The present invention relates to a prosthesis. More particularly, it relates to an acetabular prosthesis.
The efficient functioning of the hip joint is extremely important to the well-being and mobility of the human body. Each hip joint is comprised by the upper portion of the femur which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within the acetabulum in the pelvis. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the acetabulum so that the ball of the femur and the hip bone rub together causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming misshapen.
Operations to replace the hip joint with an artificial implant are well-known and widely practiced. Generally, the hip prosthesis will be formed of two components, namely: an acetabular component which lines the acetabulum; and a femoral component which replaces the femoral head. The femoral component may be total femoral head replacement in which case the component includes a head, neck and a stem which in use is inserted into the end of a prepared femur. Alternatively, where appropriate, the femoral head component may be a resurfacing prosthesis which is attached to the head of the femur once it has been suitably machined.
In an operation to insert a prosthetic acetabulum in a patient's pelvis the surgeon first uses a reamer to cut a cavity of appropriate size in the patient's pelvis. An acetabular cup is then inserted into the cavity. By “appropriate size” is meant a size which is selected by the surgeon as being the most appropriate for that particular patient. Normally, it is desirable to retain as much of the original healthy bone surface as possible.
Commercially available acetabular cups are sold in a range of sizes to suit the needs of individual patients. Generally, acetabular cups are available in sizes of from 42 mm to 62 mm diameter with 2 mm increments between neighboring sizes.
There are a number of different types of prosthetic acetabular cups. One type of cup is those made from polyethylene. They are generally cemented into the acetabulum and require only light pressure to seat them in the cement.
One alternative cup type has a polyethylene liner unit for articulation with the femur and a metal shell for insertion into the pelvic cavity. These cups with metal shells may be implanted without cement such that they rely on a jam fit between the metal shell and the patient's acetabulum. However, in some arrangements, screws may be used to secure the cup shell in position in the pelvis before the liner is applied into position. The insertion of the metal shell into the pelvis requires considerable force. As the surgeon applies this force, there is a risk that the metal shell can become damaged or deformed. There is also a possibility that during the application of the force, the shell may be moved so that it is not in the optimum alignment in the acetabulum. Often the metal shells have outer surfaces or coatings which encourage bone to grow into them over time.
With this type of prosthesis, the polyethylene liner unit is snapped or screwed into the metal shell after the metal shell has been seated in the acetabulum. Thus the inner surface of the liner forms the socket part of the joint.
More recently, ceramics have been used as an alternative to the plastics liner. In this arrangement, the metal shell, which is generally formed from titanium, is inserted into the acetabulum. The ceramic liner is then inserted into the shell.
Whilst these various prior art arrangements offer relief to patients from the pain of the worn joint, there is a continuing desire to provide prostheses which provide improved results to the patient, particularly in terms of wear. This is particularly important for younger patients where revision operations may be required if the prosthesis itself becomes worn or becomes loosened in the acetabulum. There is therefore an ongoing need for new materials and/or new structures for prostheses which more closely mimic the natural bone while minimizing the wear problems thereof.
Polyetheretherketone (PEEK) polymers have been suggested as being suitable materials for use in orthopaedic implants. This material is discussed in “Taking a PEEK at Material Options for Orthopedics”, Kinburn A., Medical Design Technology Magazine; 1 Jan. 2009 page 26ff. It is suggested that particular advantages can be obtained if the PEEK is reinforced with carbon fibres. The use of PEEK in a horseshoe-shaped cup is discussed in “Biomechanics of a PEEK Horseshoe-Shaped Cup: Comparisons with a Predicate Deformable Cup” Manley, M. T., et al Poster No 1717 at 53rd Annual Meeting of the Orthopaedic Research Society.
WO2009/097412 suggests that PEEK can be useful in a multi-layered hemispherical prosthesis. The multi-layered system comprises an outer layer formed from a porous metal and an inner layer formed from a polyaryletherketone in which the polyaryletherketone at least partially permeates the pores of the first material. The Applicants suggest that this arrangement allows the two layers to be securely bonded together. They also suggest that the prosthesis has a lower stiffness than that achieved for prior art arrangements.
An example of the use of PEEK in a prosthetic acetabular cup is described in EP 1647242. In one described embodiment, the prosthesis comprises an inner bearing surface made from a PEEK/carbon fibre composite. An outer coating is then formed on the inner bearing surface by sputtering PEEK using a plasma torch. This outer coating forms a barrier backing layer. The size of the PEEK particles at the start of the sputtering process is small and is increased during the process so that a porous structure is produced. This porous layer is coated with hydroxyapatite as a continuous layer. This coating is provided to create a barrier between the composite materials and the bone cells, provide an appropriate roughness for bone attachment and/or provide open porosity for bone cell ingrowth.
US2008/161927 describes a device for promoting fusion between first and second vertebra which comprises a first solid region formed from PEEK and a first porous region bonded to the first solid region. The first porous region has a porous PEEK architecture. A bone growth promoting material, such as hydroxyapatite, may be applied to the first porous region.
An implant formed from PEEK comprising a plurality of interconnected pores is described in WO2007/051307. Also described is a method of making the porous material in which a first material is mixed with a second material which has a melting point that is higher than the first material, heating the mixture under pressure to a temperature between a melting point of the first material and a melting point of the second material, cooling the molten mixture until it hardens and removing the second material from the first material. There is also described a method which comprises mixing a fluid material with a solid particulate sacrificial material to form a mixture, hardening the mixture and removing the solid particulate from the hardened mixture to leave a plurality of interconnected pores. Generally coarse table salt is used as the sacrificial material
Whilst these proposals go some way to addressing the problems of the prior art there is still a need for alternative, preferably improved, arrangements.